The boy, along with his mother and step-father were walking in the park, known as the soccer fields, on Cunynghame Street at 1pm on Sunday when the incident occurred.

Chifley Local Area Command Crime Manager Detective Inspector Luke Rankin said police were told the boy had been "knocked down and struck his head" after he became tangled in the leads of the two family dogs.

"The dogs [an Alsatian and a husky] were on a lead and they got loose and in some way the leads have tripped the child over," he said of the mother's report to police.

"The child had been rendered unconscious and rushed home from where emergency services were called."

A crime scene was established at the family home, with forensic police officers called in to examine the inside and outside of the home, along with an area in the park following the accident.

Police on Monday removed a number of items from the family property including several boxes of ammunition.

"We've been working to establish what's actually happened," Detective Inspector Rankin said.

"Our prominent line of inquiry is we are trying to establish if the child has injuries consistent with what the parents have told us."

The mother and stepfather are at Westmead Hospital and will be interviewed later today. 

Following the incident the child was treated by Oberon paramedics and police before being airlifted to Westmead Children's Hospital in Sydney where he remains.

"The child was injured very severely to be rendered unconscious," Detective Inspector Rankin said.

"He was in a critical condition which stabilised somewhat in the morning [on Monday]."

Police are also liasing with the NSW Department of Community Services (DOCS).

Police are now focused on potential witnesses and have already spoken to a number of children following the incident, but are seeking further witnesses.

If you have information that will assist in this investigation contact Oberon Police on 6336 1000 or Crime Stoppers on 1800 333 000.